1. Field of the Invention
The present invention relates to a process for the production of a magnetic recording disk and more particularly to a process for the production of a particulate type magnetic disk, which has a magnetic layer where a ferromagnetic powder is dispersed in a binder, suitable for high density recording involving a specific orientation method.
2. Description of the Related Art
In the art of magnetic disk, the trend is for more 2 MB MF-2HD floppy disks made of Co-modified iron oxide to be normally mounted on personal computers. However, the capacity of such a 2MB MF-2HD floppy disk is not necessarily sufficient under today""s circumstances where the amount of data to be treated has shown a sudden increase. It has thus been desired to increase drastically the capacity of floppy disks.
As a magnetic recording medium there has heretofore been widely used one obtained by applying a magnetic layer having an iron oxide, Co-modified iron oxide, CrO2, ferromagnetic metal powder and hexagonal ferrite powder dispersed in a binder to a support. Among these materials, ferromagnetic metal powder and hexagonal ferrite powder are known to have excellent high density recording properties.
Examples of magnetic disks include large capacity disks comprising a ferromagnetic metal powder having excellent high density recording properties such as 10 MB MF-2TD and 21 MB MF-2SD, and large capacity disks comprising hexagonal ferrite such as 4 MBMF-2ED and 21 MB floptical. However, these magnetic disks leave something to be desired in capacity and performance. Under these circumstances, many attempts have been made to improve high density recording properties. During the process of development, the following knowledge has been found concerning the orientation of magnetic material.
It is important that a ferromagnetic powder itself has a high acicularity ratio to realize a high coercive force due to its anisotropy in shape. It is important in a tape-like medium that the magnetic layer itself has a raised magnetic orientation in the direction according to the running direction of the head. In a rotary recording medium such as floppy disk, it is more important that the variation of output in the circumferential direction is minimized than the magnitude of output is maximized because digital recording is effected. It is thus important that the magnetic orientation in the magnetic layer is so-called random orientation free of anisotropy (i.e., orientation ratio (anisotropy in magnetic orientation in the plane of magnetic layer normally represented by the ratio of squareness ratio in a predetermined direction to squareness ratio in the direction perpendicular to that direction as a measure) of close to 1).
In order to realize high density recording, it is important to reduce the particle size of the magnetic powder further. However, a problem has arisen during the development of a large capacity floppy disk having a face recording density of greater than 0.2 Gbit/inch2 that as the particle size of the magnetic material decreases, there occur more noises. In order to inhibit the generation of noises, it is necessary that the agglomeration of magnetic particles to each other be eliminated and the content of vertical magnetization components (vertically magnetized components) be reduced. In order to meet these requirements, orientation becomes a great factor. Further, as the particle size of the magnetic powder decreases, the dispersion of the magnetic powder in the binder during the preparation of the magnetic layer coating compound is made difficult, making it difficult to obtain the desired orientation even after the application of the magnetic layer coating compound to the support.
As techniques for random orientation of magnetic powder in a magnetic layer there have heretofore been proposed the following approaches.
JP-A-6-36261 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a recording medium having a lower non-magnetic layer and a thin magnetic layer obtained by ATOMM (Advanced Super Thin Layer and High Output Metal Media Technology) which comprises performing random orientation under wet conditions, and then performing oblique orientation to attain an in-plane and vertical orientation ratio of not smaller than 0.85 and a vertical squareness ratio of from 0.3 to 0.65. In some detail, a magnetic disk which can give a uniform and high circumferential output and excellent overwrite properties as compared with those obtained by the conventional orientation-free processing and a process for the production thereof are provided. However, the ferromagnetic powder used in the examples is as large as 0.20 xcexcm and 195 angstrom as calculated in terms of major axis length and crystalline size, respectively.
JP-A-63-148417 discloses a process which comprises applying an alternating magnetic field while the magnetic layer is undried to perform random orientation, wherein the intensity of magnetic field is from 1/10 to 1/1 of Hc of the ferromagnetic powder and the frequency of the alternating magnetic field is from 1/10 to 1/1 of the coating speed. In accordance with this approach, the range of the intensity of magnetic field is predetermined while the relationship between the frequency and the coating speed is predetermined within a predetermined range. In this arrangement, the orientation ratio of 1 can be continuously maintained to enable stable random orientation. Although the approach disclosed in the above cited patent can perform three-dimensional random orientation, a sufficient S/N ratio cannot be secured. Further, the magnetic materials used in the examples of the above cited patent are xcex3-Fe2O3 and Co-containing xcex3-FeO3 having Hc of from 240 to 600 Oe. However, a particulate magnetic material having a high coercive force (Hc) required for high density recording (particularly a magnetic metal powder or hexagonal tablet-like hexagonal ferrite having a high as value) is subject to agglomeration. Thus, it is necessary to take a measure for inhibiting noises.
JP-A-1-248321 discloses a process which comprises performing random orientation shortly after vertical orientation. This invention contemplates the combined use of vertical orientation and random orientation that makes it possible to provide a medium having little mechanical orientation, a high orientation ratio and good modulation properties (variation of reproduced output in the circumferential direction on the magnetic disk). However, this approach is disadvantageous in that since vertical orientation has been once effected, the vertical component of magnetization, which factor is the target of control in the present invention, tends to grow. Further, in the examples of the above cited patent, a magnetic powder having a particle size as large as 0.25 xcexcm as calculated in terms of major axis length and an acicularity ratio as large as 10 is used. Such a magnetic powder having a relatively large particle size and large acicularity ratio can easily be arranged parallel to the surface of the magnetic layer. However, these examples leave something to be desired in attaining a sufficient orientation because a magnetic metal powder having a small major axis length and a small acicularity ratio must be used to secure a high S/N ratio essential for high density recording.
JP-A-63-171428 discloses a process which comprises subjecting a particulate ferromagnetic material to orientation in a magnetic field in a predetermined direction, and then subjecting the ferromagnetic material to orientation in a weak alternating magnetic field in the direction almost perpendicular to that of the former magnetic field for random orientation. However, since the magnetic powder used in the examples of the above cited patent is xcex3-Fe2O3, which has a small magnetizability than magnetic metal powder, sufficient electromagnetic properties cannot be obtained.
JP-A-1-105328 discloses a process which comprises subjecting a magnetic material to crosswise orientation, and then subjecting the magnetic material to uniform deorientation in a solenoid to which an alternating magnetic field is applied. In the above cited patent, it is certain that the in-plane orientation ratio can be improved. However, there is no definition on the inhibition of agglomeration of magnetic particles and vertical magnetization. Thus, this approach leaves something to be desired in attaining a high S/N ratio.
JP-B-5-53009 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent applicationxe2x80x9d) discloses a process which comprises arranging a plurality of bar-shaped orientation magnets apart from each other at a certain interval in the direction of conveyance of the support such that the polarity of magnets differ from that of adjacent magnets and these magnets are disposed oblique to the conveying direction and face in the direction opposed to that of adjacent magnets, where by random orientation is allowed. In this arrangement, a good modulation can be certainly obtained, attaining a high orientation ratio. However, since the magnetic powder used in the examples of the above cited patent is xcex3-Fe2O3, which has a small magnetizability than magnetic metal powder, sufficient electromagnetic properties cannot be obtained. Further, since there is no random orientation apparatus using an alternating magnetic field, modulation is deteriorated unless the intensity of magnetic field of the bar-shaped orientation magnet is predetermined to be not greater than 50 Oe. A particulate magnetic metal cannot be sufficiently oriented in such a low magnetic field.
As mentioned above, there have been disclosed many techniques for performing so-called random orientation free from anisotropy as magnetic orientation in the magnetic layer in the production of magnetic disk. However, no effective means have been found for realizing desired orientation and reducing noises even by reducing the particle size and acicularity ratio of magnetic powder to increase the coercive force thereof for the purpose of performing high density recording.
Therefore, an object of the invention is to provide a production process suitable for the provision of a large capacity magnetic disk suitable for digital recording having good electromagnetic properties, S/N ratio and modulation which can provide a desired orientation and eliminate agglomeration of magnetic particles to each other to reduce noises even if the particle size and acicularity ratio of the magnetic powder are reduced to enhance the coercive force thereof.
The inventors made extensive studies. As a result, it was found that the use of the following production process makes it possible to accomplish the object of the invention, i.e., obtain a desired orientation, eliminate the agglomeration of magnetic particles to each other and reduce the content of vertically magnetized components to reduce noises, thus obtaining a large capacity magnetic disk suitable for digital recording having good electromagnetic properties, S/N ratio and modulation. Thus, the present invention has been worked out.
In other words, the present invention has the following constitutions.
(1) A process for producing a magnetic disk having a randomly oriented magnetic powder, which comprises: applying a magnetic coating solution containing at least a magnetic powder to a web that is being continuously conveyed, so as to prepare a magnetic layer; applying a first external magnetic field to the magnetic layer while the magnetic layer is wet; and applying a second external magnetic field to the magnetic layer while the magnetic layer is wet, wherein: the first external magnetic field is applied with a set of a first pair of same-pole-opposed magnets with the web interposed therebetween and a second pair of same-pole-opposed magnets with the web interposed therebetween; the first and second pairs are provided on the same plane of the web and on two equal sides of an isosceles triangle so that a perpendicular line dropped from a base of the isosceles triangle forms a line perpendicular to a conveying direction of the web; and the second external magnetic field is an alternating magnetic field, and is applied with a pair of magnets with the web interposed therebetween; the pair of magnets being provided on the same plane of the web, and in a direction perpendicular to the conveying direction of the web.
(2) The process for producing a magnetic disk according to Clause 1, wherein the same-pole-opposed magnets in each of the first and second pairs are a pair of permanent magnets, and the intensity of magnetic field in a center of a gap between the opposed permanent magnets in each of the first and second pairs is from 1/3 to 10 times the coercive force of the magnetic layer.
(3) The process for producing a magnetic disk according to Clause 1, wherein the random orientation by the application of an alternating magnetic field is effected in such a manner that the intensity of the second external magnetic field is from 1/40 to 10 times the intensity of the first external magnetic field.
(4) The process for producing a magnetic disk according to Clause 1, wherein the magnetic powder is a ferromagnetic metal powder having Hc of from 114 to 280 kA/m (from 1,400 to 3,500 Oe) and an average major axis length of from 0.01 xcexcm to 0.18 xcexcm.
(5) The process for producing a magnetic disk according to Clause 1, wherein the magnetic powder is a hexagonal ferrite magnetic material having an average plate diameter of from 0.01 xcexcM to 0.1 xcexcm.
In accordance with the foregoing production process, even when as a magnetic powder there is used one having a small particle size and acicularity ratio and a great coercive force, a desired orientation ratio can be obtained, a vertically magnetized component can be obtained, and the agglomeration of magnetized particles can be relaxed, making it possible to realize the reduction of noise.
The restriction of the orientation ratio (Or) in the plane of the magnetic layer and the squareness ratio (SQn) in the direction perpendicular to the plane of the magnetic layer contributes to the reduction of S/N ratio and modulation. The restriction of the particle size, acicularity ratio and coercive force (Hc) of the magnetic material is needed to secure electromagnetic properties, particularly output.